Talked to catlee, he looked at the logs and found that at 11:37:20 we got a 500 error from hg.m.o/try
/builds/buildbot/build_scheduler/master/twistd.log.5:2012-07-17 11:37:20-0700 [HTTPPageGetter,client] http://hg.mozilla.org/try has been reset
He suspects this is the root cause and subsequent builds should be fine.

Ah, due to bug 770811, most of you won't be able to load that page easily :-(
The push in question:
amarchesini@moco – Fri Jul 20 03:23:21 2012 (UTC+1)
eaf06d352463 Andrea Marchesini – try: -b do -p all -u all -t none
(There were many more like it, this was just the most recent push on which it occurred)

I've cancelled all doubled up running and pending tests (approx 100, all Windows).
There were over 30 pushes last night (UTC) that had doubled up builds; one plus side is that once this is resolved at least bug 772458 gets an easy win :-)